1. Field
Embodiments of the present disclosure relate to laser projection systems and, more specifically, to laser projection systems that reduce the appearance of speckle in a scanned laser image.
2. Technical Background
Speckle may result whenever a coherent light source is used to illuminate a rough surface, for example, a screen, wall, or any other object that produces a diffused reflection or transmission. Particularly, a multitude of small areas of the screen or other reflecting objects scatter light into a multitude of diffracted beams with different points of origination and different propagation directions. Speckle causes high spatial frequency noise in the projected image. At an observation point, for example in the eyes of an observer or at the sensor of a camera, these beams interfere constructively to form a bright spot, or destructively to form a dark spot, producing a random granular intensity pattern known as speckle. Speckle may be characterized by grain size and contrast, usually defined as a ratio of standard deviation to mean light intensity in the observation plane. For a large enough illuminated area and a small enough individual scattering point size, the speckle will be “fully developed,” with a brightness standard deviation of 100% if the diffuser is not depolarizing light and of about 71% when the diffuser is depolarizing light. If an image is formed on the screen using a coherent light source such as laser beams, such granular structure will represent noise or a serious degradation of the image quality. This noise presents a significant problem, particularly when the projector is used to display high spatial frequency content, such as text.
Speckle may be reduced by inserting some moving diffusing surfaces into the optical path of the scanned laser beam to scramble the phase of the light hitting the screen. The moving diffusing surface modifies the shape of the speckle pattern as a function of time and, if the diffusing surface is moving fast enough, all those speckle patterns are averaged since humans integrate images typically over durations in the order of 50 ms.
Although rapidly moving the diffuser provides speckle reduction, it requires expensive and complicated mechanisms to move the phase mask laterally at a relatively high speed. Further, a moving diffuser requires the use of focus mechanisms as well as lenses possessing a high numerical aperture and a high field of view, which adds significant complexity, cost and size to the system. Therefore, the use of a moving diffuser presents some significant drawbacks when implementing such approaches in small size projectors.